This invention relates to a process for the purification of epoxy resins.
Epoxy compounds or "epoxy resins" are employed primarily as binders in paints and varnishes, as cast resins for the production of laminates, as insulating materials for electric conductors, and as adhesives and thus are of great commercial significance.
These compounds are obtained almost exclusively by glycidylation, i.e., by reacting di- or higher-functional compounds, such as, for example, polyalcohols, polyphenols, polymercaptans, polycarboxylic acids, or amines with, in most cases, excess epihalohydrin (or .beta.-methylhalohydrin), employing agents which split off hydrogen halide, such as, e.g., alkali metal hydroxide. Generally, epichlorohydrin is employed as the epihalohydrin and the excess epihalohydrin as the reaction solvent.
The functional groups --OH, --SH, --COOH, .dbd.NH of these compounds which are reacted with epichlorohydrin can be present not only on a low-molecular residue, such as, for example, arylene or alkylene, but also on a high-molecular organic residue, such as, for example, a polyester or polyurethane chain.
To conduct such glycidylations, for example, as described in German Unexamined Laid-Open Applications DOS No. 1,643,777 and DOS No. 1,816,096, and as generally utilized except in the glycidylation of several polyalcohols, catalysts are employed, viz., tertiary sulfonium, quaternary ammonium, and phosphonium compounds and/or compounds which are converted into these substances under the reaction conditions, e.g., secondary sulfides, tertiary amines, and tertiary phosphines. Such catalysts are known, for example, from the German Patents or Laid-Open Applications Nos. 1,211,177, 1,816,096 and 1,911,478.
By azeotropic distillation with the excess epihalohydrin employed as reaction solvent, the largest portion of the water produced during the reaction is removed from the reaction mixture and the moist (about 3% water content) epihalohydrin is recycled into the reaction mixture.
The reaction yields a mixture consisting essentially of a salt suspension in a solution of the epoxy compound and the catalyst in the excess moist epihalohydrin, the salt being produced during the formation of the glycidyl groups by reaction of the agent splitting off hydrogen halide from the intermediately produced halohydrin. In general, this salt is an alkali halogenide, in most cases, sodium chloride.
To produce the desired epoxy product in pure form, the precipitated salt, the catalyst, and the excess moist epihalohydrin must be removed from this reaction mixture.
The salt can be removed by simple filtration but the catalyst, which is dissolved in the reaction mixture, cannot. Accordingly, in the conventional work-up, after separating the alkali halogenide by filtration or centrifugation, the catalyst is extracted from the mixture with water before the epoxy resin can be isolated by concentrating the resulting solution by evaporation of the excess epihalohydrin.
The extraction of the catalyst with water is, however, complicated since the organic phase has a greater density than the aqueous phase, thus requiring special equipment, and since emulsions are readily formed which make phase separation difficult. To avoid these emulsions, it is suggested, for example, in German Pat. No. 1,643,777, column 10, either to extract at elevated temperatures or dissolve the epoxy resin in a solvent, e.g. benzene after the alkali halogenide has been filtered off and the excess epichlorohydrin has been distilled off. Although this facilitates the extraction, the epoxy resin solution must be concentrated twice in this process.
It has furthermore been proposed to add aqueous ethanol to prevent the formation of an emulsion of the aqueous phase (Dutch Laid-Open Application No. 6,901,372), but this measure is not effective in the case of several higher-molecular epoxy resins. In the extraction of a 30% solution of a diglycidyl polyester with a molecular mass of 1,000 in epichlorohydrin or in ethyl acetate, the time required for phase separation is reduced from about 24 hours to 4-8 hours when using a 40% aqueous ethanol solution instead of pure water. However, in the usual case of repeated extraction, this time is still too long for a commercial process.
In any event, the removal of the catalyst, required to ensure a sufficient shelf stability of the epoxy resin, is effected in accordance with the state of the art by a repeated extraction with water and/or aqueous solutions, either before or after the separation of the alkali halogenide and before or after the removal of the epihalohydrin or another auxiliary solvent from the reaction mixture obtained during the introduction of the glycidyl groups. These state of the art processes are cumbersome, difficult in part due to poor phase separation, and require large capital expenditures for apparatus to conduct them.
Czechoslovak Pat. No. 119,415 claims a process for refining technical grade epoxy resin by the removal therefrom with adsorbent of trace amounts of residual inorganic salts, water unsoluble macromolecules containing nitrogen and sulfur, which cannot be extracted by water, organic chelates of heavy metals and aluminum and degradation products of the epoxy resin or epichlorohydrin. Since no reference is made to the reaction catalyst and the starting epoxy resin is described as technical grade, it is apparent that the catalyst and the excess epichlorohydrin conventionally employed have been removed from the epoxy resin in a conventional manner, e.g., by washing with water and then placing the washed resin under a vacuum. In any event, from the brief description of the process immediately preceding the examples, it is apparent that prior to the treatment with adsorbent, the epoxy resin has been dissolved in toluene or benzene, freed of water (and apparently also reaction catalyst and excess epichlorohydrin) and the precipitated salts separated by filtration. None of these preliminary steps are required in applicants' process.
It is an object of this invention to provide a process for the removal of the catalyst which avoids the aforedescribed disadvantages of the prior-art processes. Other objects will be apparent to those skilled in the art to which this invention pertains.